The English term Cross Luminance [ ˈkɹɒs ˈluːmɪnəns ], colloquially also Dot crawl , describes fine, moving structures in a color television picture that do not exist in the original scene , whereby color information is incorrectly interpreted as brightness. These occur in strongly colored surfaces and especially on vertical colored edges, where two strongly colored surfaces but with opposite shades of color border one another - if, for example, a boxer dressed in red is shown in a blue boxing ring, the effects appear quite clearly on the edges of his clothing. The complementary case (brightness erroneously expressed as color) is called cross color .
The cause of cross luminance is that parts of the color carrier signal (color) are misinterpreted as a brightness signal (luminance) in the receiver.
PAL vs. NTSC
In favor of the better color stability, the effect is clearly more pronounced with PAL than with NTSC , since with NTSC a shift of a whole half line is created for the color subcarrier, with PAL however only a quarter line. In order to divert the hue errors, which are more common with NTSC, into the less noticeable saturation area, the phase position of V (= red signal of the picture) is constantly switched with PAL, which acts like a renewed modulation with half the line frequency (hence quarter lines instead of half lines with PAL). Unlike NTSC a half line does not arise in the same room so a carrier with I (Cyan Orange ratio) and Q (magenta-green ratio) component, but two different (for U '(blue) and V' Red ) of a quarter line each.
If one were to choose a full half-line offset for the color subcarrier, as is the case with NTSC, one of the two components would always be exactly on the luminance spectrum. Therefore, with PAL, the entire color subcarrier modulation is only shifted by a quarter line. In the gap in the luminance spectrum, in the above a further carrier lag (for transmission of the color signal IQ), are now two (V-Red and U-blue). The result is that the carriers are physically closer to one another and therefore cross talk more often.
Cause and occurrence
Cross-luminance and cross-color disturbances occur when the brightness signal (luminance) and the modulated chrominance signal are added together and transmitted on one line and the spectra overlap (the latter is usually always the case). The effect occurs when this happens at any point in the entire transmission path from production to display on a screen.
- RF transmission
- Transmission via analog television standard: The underlying PAL, SECAM and NTSC color systems are the source of this interference.
- Transmission via DVB or VSB: no interference
- Baseband transmission
- FBAS video transmission via Cinch, BNC, SCART or any other cable: The underlying PAL, SECAM and NTSC color systems are the source of this interference.
- S-Video transmission via Hosiden or SCART: no interference if both signals remain separate
- RGB transmission via Cinch, BNC, SCART, Sub-D: no interference
- Digital transmissions via DVI, HDMI, DisplayPort: no interference
- Digital encodings (DVC, MPEG-2, MPEG-4 AVC, MPEG-4 HEVC) with any transmission path (USB, DV, LAN, W-LAN) and storage media: no interference
Cross Luminance can be significantly reduced nowadays with comb filters built into many video recorders , especially the newer digital comb filters are quite successful in this regard. However, simple implementations can lead to artifacts , e.g. B. lead strings of pearls.
2D comb filter
- Only takes into account the current image ( horizontal and vertical dimensions, therefore 2D)
- No disturbances if the lines above and below are similar
- remaining disturbances on horizontal edges
3D comb filter
- Takes into account the current and one or two previous images ( horizontal , vertical and temporary dimensions, therefore 3D)
- in addition, disturbances at horizontal edges in static image sections are reduced
Cross-luminance artifacts and its complementary error, cross color, can most easily be reduced by software after digitization, as complex algorithms can be implemented. The freeware plug-in Dotcrawl for VirtualDub , intended for NTSC material, is suitable for reducing these artifacts, for the scripting language AviSynth there are also the slightly better working filters Guavacomb and DeDot .
- DotCrawl Comb Filter for VirtualDub on the homepage of the filter author
- Guavacomb for AviSynth on AviSynth.org ( Memento from September 9, 2013 in the Internet Archive )
- DeDot_YV12 (only works in the YV12 color space, related to YUV ) for AviSynth
- Schmidt, Ulrich, "Professionelle Videotechnik", 4th updated edition, Springerverlag, Heidelberg, 2005
- Mahler, Gerhard, "The basics of television technology", Springerverlag, Heidelberg, 2005
The reverse case (brightness erroneously expressed as color) is called cross color .